For many applications, and in particularly in consumer electronics devices, the relatively large and heavy cathode rate tube has been replaced by a flat panel display type, such as a liquid crystal display (LCD), plasma, or organic light emitting diode (OLED). A flat panel display screen contains an array of display elements. Each element is to receive a signal that represents the picture element (pixel) value, such as an intensity value of a particular color, or a gray scale value, to be displayed at that location of the screen. This pixel signal may be applied using a transistor, e.g. a pixel TFT that is coupled to and may be said to be integrated with the display element. The transistor may act as a switch element. It has a carrier electrode that receives the pixel signal, and a control electrode that receives a gate or select signal. The gate signal may serve to modulate or turn on and turn off the transistor so as to selectively apply the pixel signal to the coupled display element.
Typically, thousands or millions of copies of the display element and its associated switch element (e.g., an LCD cell and its associated control transistor) are produced in the form of an array, on a substrate such as a plane of glass or other light transparent material. The array is overlaid with a grid of data or source lines, and gate lines. The source lines serve to deliver the pixel signals to the carrier electrodes of the control transistors, and the gate lines serve to apply the gate or select signals to the control electrodes of the transistors. Each of the source lines is coupled to a respective group of display elements, typically referred to as a column of display elements, while each of the gate lines is coupled to a respective row of display elements. This type of active matrix allows individual display elements to be driven with their respective pixel signal values independently, using a raster scan approach. To do so, each gate or select line is coupled to a gate line driver circuit that is controlled by appropriate timing or clock signals so that it is driven in a vertical shift register fashion. In contrast, the source lines are driven by source line driving circuitry that operates in a horizontal shift register fashion. Together, the line-by-line scanning of the display element array can be achieved.
The source lines are coupled to a source line driver circuit that is within a display driver integrated circuit (or simply display driver IC). The latter translates incoming digital video or digital pixel values (for example red, green and blue digital pixel values) into analog pixels signals that have the appropriate timing, voltage swing and fan-out. The source line driver circuitry performs any needed voltage level shifting or amplification to produce a pixel signal with the needed fan-out or current capability, on each source line.
To reduce overall display system cost, the display driver IC has been encased and installed directly on the light transparent panel that is part of the display screen, rather than being reached via a flex circuit in an off-panel location on a printed circuit board. In addition, the gate line driver circuitry has typically been implemented using essentially TFT on-glass devices, rather than as part of the display driver IC which is built on a separately manufactured microelectronic semiconductor substrate using for example a metal oxide semiconductor (MOS) fabrication process.
To help further reduce the costs of the system and in particular that of the driver IC, attempts have been made to reduce the number of external signal pins of the driver IC. This helps prevent the driver IC from becoming too large. This can be achieved by adding a demultiplexing (demux) function to the source line driving circuitry. The demux in effect allows a single analog external pin of the driver IC, which provides analog pixel signals, to be shared by several source lines or “channels” of the display element array. For example, in a red, green and blue (RGB) LCD panel, a 1:3 demultiplexing approach can be used to supply pixel signals to the three channels, where a group of three source lines are fed by three outputs of a demultiplexer circuit, sequentially from a single input of the demultiplexer circuit. The single input sequentially receives (as controlled by buffers in the display driver IC) red, green and blue analog pixel values. Such a demux circuit has been implemented as a number of single transistor, N-channel TFTs that are operated as switches under control of timing circuitry that is in the display driver IC.